Standardized cotton swab sampling with nested quantitative polymerase chain reaction is effective for diagnosing ordinary scabies.

Background: Low sensitivity of the PCR assay for diagnosing scabies has been noted because of the difficulty in obtaining tissue containing Sarcoptes scabiei DNA.

Aim: To evaluate nested real-time quantitative PCR (nRT-qPCR) with nonexpert-dependent standardized cotton swab sampling (CSW) as a tool for diagnosing scabies.

Methods: All patients underwent dermoscopic and microscopic examination (MS) with scraped sampling (Sc). Patient samples were acquired with a single, dry swab rubbed across the flexor areas of both wrists as well as the eight interdigital spaces and on any suspected scabies lesions. nRT-qPCRs were performed with Sc and CSW samples.

Results: Out of 125 patients with suspected scabies, 120 patients were sampled, and 57 were positive (positive with: MS n = 53; nRT-qPCR with Sc n = 52; nRT-qPCR with CSW n = 46) and 63 were negative for scabies. The sensitivities of these tests were 93.0%, 91.2% and 80.7%, respectively, which were not different statistically (P > 0.05). However, upon subsequent monitoring after treatment, the sensitivity of nRT-qPCR with CSW was only 36.6%, which was significantly lower than 83.0% for MS and 92.7% for nRT-qPCR with Sc (P < 0.001). The obtained sequences showed 97%-100% homology with scabies sequences deposited in GenBank.

Conclusion: CSW with nRT-qPCR shows sensitivity close to MS with scraping performed by experts for diagnosing scabies in an outpatient setting, but not for post-treatment monitoring. CSW with nRT-qPCR may be useful for physicians unfamiliar with a traditional diagnostic method, and for screening an outbreak in community facilities.